The Science of Tropical Cyclone Sandy--Live Chat, October 30 [Transcript]

Join us below at 1 P.M. Eastern time on Tuesday, October 30 for a live 30-minute online chat with climate scientist Noah Diffenbaugh of Stanford University, who will discuss Sandy--the tropical cyclone that has caused 20 deaths and left more than seven million people in the U.S. Northeast without power. Diffenbaugh will answer questions about the storm as well as the role of climate change in severe storms and this storm. We invite you to post chat questions in advance in the comments below.

CHAT TRANSCRIPT

Lloyd
Welcome to the Scientific American chat. We'll be live for the next 30 minutes w/Stanford University climate scientist Noah S. Diffenbaugh. Diffenbaugh is an expert on climate modeling and prediction.

Lloyd
I'm Robin Lloyd, Scientific American's news editor. We welcome questions from all of you participating in this chat.

Diffenbaugh
Hi everyone! I'm excited to join the discussion!

Lloyd
Dr. Diffenbaugh, how surprised were you by the intensity of this storm? Has your research done anything to predict such intensity?

Diffenbaugh
This is certainly a rare storm, largely because of the co-occurrence of a tropical and extratropical storm. The co-occurrence is what is so rare. That co-occurrence is really a result of variability in the atmosphere and ocean.

Lloyd
So the extratropical was the front that came down from the Arctic? And the tropical storm was the one that came up from the south via the Caribbean?

Diffenbaugh
The extra-tropical storm coming 'down' from Canada interacted with the tropical storm to 'steer' the tropical storm. Here is a nice discussion by Prof. Adam Sobel of Columbia: http://bit.ly/TLbDdo

Diffenbaugh
There is also a high pressure center over the North Atlantic interacting. My view is that this storm can teach us a lot about the vulnerability of people and ecosystems to severe storms and also about the physics of these storms because we have a lot of instrumentation now.

But in terms of the frequency of these kinds of storms, we have strong evidence that storm surge will increase (on average) with sea level rise. We also have strong evidence that the precipitation intensity is likely to increase with global warming. But whether global warming will cause more of these 'perfect' storms is not something that we know.

Josue Viv
What made NHC decide a couple hours before landfall that this was no longer a tropical system, even though it still had a warm core?

Diffenbaugh
The hurricane scale is based on the physical properties, not the expected damage. A lower category hurricane can cause severe damage, particularly if it strikes a vulnerable location. So once the physical properties are not on the scale, the storm is 'downgraded.’

Markfischetti
Some scientists say the storm 'down' from Canada was Jet Stream influenced by NAO, or 'blocking pattern. Comments?

Diffenbaugh
Markfischetti: Yes, the 'long wave pattern' influences extratropical storm track. The pattern of high and low pressure in the atmosphere is what creates that long wave pattern, and the pressure pattern in influenced by the NAO. This storm is a case where the particular configuration of the atmospheric pressure played a key role in the storm turning westward and making landfall in the Northeast.

Diffenbaugh
Josue Viv:
Do you see the gulf stream waters staying warmer than usual as a factor of stronger tropical systems affecting the northern east coast? The sea surface temperature response to global warming is a key question. We might expect that as ocean temperatures warm at higher latitudes more tropical storms could persist farther north. However, the strength of shearing winds in the atmosphere is also critical. The relative influence of increasing ocean temperatures and changes in shear is a key area of current research and there is not a definitive understanding currently.

John Bailo
I still don't understand the origin of the low pressure. And are you saying that high pressure somehow presses down on the oceans, normally?

Diffenbaugh
John Bailo and Pmagn: The storm surge is a result of the high winds and low pressure. The exact orientation of the storm track as it makes landfall is critical for where the peak surge occurs. In fact, Hurricane Katrina could have been much more damaging, but the peak surge was not centered on the most populated area.

Markfischetti
Good points about ocean warmer farther north and Gulf Stream closer to coast; mo' research needed #sciamchat

Diffenbaugh
Markfischetti: We would usually see a clockwise storm track in the northern hemisphere due to the Earth's rotation, and the jet stream (moving west to east) ‘steering' toward the west is a big part of what has made this event so rare. The effect of global warming on this sort of co-occurrence is essentially unstudied.

Diffenbaugh
At this stage, my view is that the only scientifically defensible statement is that variability is the main cause. THAT SAID, there is strong evidence that when such a storm occurs, global warming will increase the odds of high storm surge and heavy precipitation.

In_cue36
It seems that our abilities to predict weather are well in hand. As these storms occur outside of the norms, will they be harder to predict?

Diffenbaugh
In_cue36: The forecasts for the direction, intensity and location of landfall were clear for the past several days. Within a few days the models have high accuracy, in part because we have a rich observing system, in part because we understand the short-term physics well. Without satellite observations and very brave hurricane hunters, predictions would be much less accurate.

Sue Ann Bowling
So far the prediction on this one has been astonishingly good.

Diffenbaugh
Sue Ann Bowling: Yes, prediction is a key component of avoiding disaster. Then how people respond to the predictions/warming also plays a key role. In the case of both this storm and Katrina, the predictions were accurate days in advance.

Josue Viv
Some models predict that as water temps rise during the century so will wind shear hindering development. However, the systems that do develop will be more massive, but this just doesn't seem to be the case.

Diffenbaugh
Josue Viv: Both global warming and variability are playing a role....

Lloyd
Thanks everyone we are about finished up here. Stay tuned for further chats on Sandy.

Lloyd
Thanks especially to Dr. Noah Diffenbaugh for his expertise and for well-informed answers to our questions.

Diffenbaugh
I'm getting the heads up that we are out of time - I have to go teach my freshman seminar! Thanks to everyone for joining - these are great questions!